Created ControlNet Web API (markdown)

ControlNet-Web-API.md

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+This page is a guide on how to use ControlNet's Web API.
+
+You need to pass parameters to ControlNet script, when you use [A1111's txt2img/img2img generation API](https://github.com/AUTOMATIC1111/stable-diffusion-webui/wiki/API).
+
+Here is an example payload in Python:
+```python
+import base64
+import cv2
+import requests
+
+def read_image(img_path: Path) -> str:
+    img = cv2.imread(str(img_path))
+    _, bytes = cv2.imencode(".png", img)
+    encoded_image = base64.b64encode(bytes).decode("utf-8")
+    return encoded_image
+
+# Note: You can no longer pass file path in payload as in sd-webui-controlnet.
+base64image = read_image("D:/foo.png")
+base64mask = read_image("D:/bar.png")
+
+unit1 = {
+    "image": base64image,
+    "mask_image": base64mask,
+    "control_mode": "Balanced",  # "Balanced", "My prompt is more important", "ControlNet is more important" 
+    "enabled": True,
+    "guidance_end": 1,
+    "guidance_start": 0,
+    "pixel_perfect": True,
+    "processor_res": 512,
+    "resize_mode": "Just Resize",  # "Just Resize", "Crop and Resize", "Resize and Fill"
+    "threshold_a": 64,
+    "threshold_b": 64,
+    "weight": 1,
+    "module": "canny",
+    "model": "sd15_canny.pth",
+    "save_detected_map": True,
+    "hr_option": "Both",  # "Both", "High res only", "Low res only"
+}
+unit2 = ...
+unit3 = ...
+payload = {
+    "alwayson_scripts": {"ControlNet": {"args": [unit1, unit2, unit3]}},
+    "batch_size": 1,
+    "cfg_scale": 7,
+    ... (other parameters)
+}
+
+resp = requests.post("http://localhost:7860/sdapi/v1/txt2img", json=payload)
+imgs = resp.json()["images"]
+```